In order to test telephone loops, whether for fault diagnosis, preventive maintenance purposes or even to compile statistical information about loop characteristics, three basic functions are required, namely: access, test and communication. These three basic functions can readily be identified for any manual or automatic testing system. For instance, within each system, there are mechanisms for gaining control of a loop to be tested, for connecting to it and for directing appropriate testing activities. Moreover, a two-way communication path exists between testing personnel or equipment interfaces so that selected test activities may be initiated, coordinated and the results collected for analysis. Oftentimes, an automated central controller determines the testing pattern and analyzes results via interpretive algorithms.
One such computer-based system has been described in an article entitled "The Evolution of the Automated Repair Service Bureau with Respect to Loop Testing", published in the Conference Record of the International Symposium on Subscriber Loops and Services, March, 1978, pages 64-68 as authored by O. B. Dale. The Automated Repair Service Bureau (ARSB), which supports loop maintenance operations, includes the following maintenance functions: receiving trouble reports from customers; trouble report tracking; generating management reports; and real-time loop testing and fault diagnosis. Thus, within the ARSB framework, there is provided a rapid, convenient method for testing and analyzing test results automatically at the time of customer contact as well as on demand during repair procedures.
In order that the subject matter of the present invention may be elucidated, it is important to elaborate on the ARSB architecture and the capabilities of the above-mentioned testing arrangement within this architecture. The information presented by this overview is set forth in the above-mentioned reference as well as in an article entitled "Automation of Repair Service Bureau", Conference Publication No. 137 of the International Symposium on Subscriber Loops and Services, May, 1976 as authored by R. L. Martin. FIG. 1 indicates that the conventional ARSB comprises a tree-like structure with four major levels. At Level 1 of the tree is a data storage computer (200) which maintains a master data base of up to five million customer line records; the information on these records includes data as to equipment terminating the loop, loop composition, customer telephone number, and so forth. Level 2 is composed of an array of front end (FE) computers (220,221), each of which manages the bulk of the trouble report processing for about 500,000 lines. The users of the system, typically maintenance and craft personnel of the telephone company deploying the ARSB, interact with the system at this level. Level 3 is an array of control computers (240,241) that control access and testing and provide analysis of test results. Level 4 comprises loop testing frames (250,251) which perform the loop accesses and actual test measurements via test trunk connections to switching machines located in geographically-dispersed central offices.
Test request from users are received and supervised by the FE computers and then performed by algorithms in the control computers and circuitry in the loop testing frames. The tests conducted are based on adaptive algorithms that compose test scripts in real time as a function of the electrical characteristics of the customer's equipment in the idle state. The data used are extracted from the data storage computer and then provided by the FE computers at the time the test request is generated. As testing on a customer's loop proceeds, the test script is continually being revised to reflect the knowledge of the loop which has been gained from the test results. The final test results and analysis are formatted for display to the user by the requesting FE computer. Varying levels of display detail, based on the technical sophistication of the user, are provided.
The loop testing subsystems of the ARSB were arranged to provide an area-based (about 1 million loops) system in order to expedite its introduction and mitigate cost to users. As a result, not all of the testing functions of the standard pre-ARSB facility, known as the Local Test Desk (LTD), were incorporated. For instance, the LTD continued to be used for interactive testing between testers at the LTD and field repair craft. The loop testing subsystem could not be utilized to maintain a connection to the loop under test for a prolonged duration, nor could field repair personnel be guided through a series of steps to diagnose, locate and correct a fault. In short, the loop subsystem was effective only in screening troubles and performing pre-dispatch and post-dispatch testing. Also, not every type of terminal equipment could be tested. For instance, coin telephone features were precluded from testing. Moreover, because the LTD operated within the same environment as the ARSB, the LTD was considered a backup during temporary outages of ARSB so there was no need for redundancy or fail-soft operation in the testing system. Finally, the area-oriented system was not cost effective for single wire centers serving only a few thousand lines.
With the above background, the significant limitations and deficiencies of the conventional ARSB testing system, including those emphasized above, may be summarized as follows: (1) no interactive testing capability with field craft personnel nor customers; (2) inability to test coin telephone stations including such conditions as off-normal totalizers, stuck coin conditions, coin collect and coin return circuitry, and loop-ground resistance; (3) impossible to test and talk over the same test connection; (4) no single- and double-sided resistive fault sectionalization capability; (5) no ability to apply metallic or longitudinal pair identification tones; and (6) no capability to control and monitor concurrent testing operations from a single work station.
Besides the ARSB approach, numerous other automated, but less complex, approaches have been employed to effect loop testing. Typically these have focussed on specialized problem areas, such as rapid-scan procedures to verify the accuracy and quality of splicing operations or simplified checks on easily quantifiable loop parameters like loop insulation resistance or loop impedance at a given frequency for preventive maintenance purposes.
Other automated approaches, with a sophistication comparable to the ARSB approach, have been developed for the purpose of diagnostic testing. One representative prior art system is disclosed in U.S. Pat. No. 4,139,745 issued to Ashdown et al on Feb. 13, 1979. Broadly speaking, the system comprises control means having a programmed digital computer and associated memory, a line test network, at least one user station and an interface for interconnecting these elements and one or more telephone exchanges and the plurality of telephone lines extending from such exchanges.
The line test network is responsive to the digital computer and includes means for generating a plurality of signals for a test cycle. During a cycle, besides DC and noise measurements, AC signals are applied to the three-wire line comprising the tip-ring-ground conductors and longitudinal and metallic response signals are measured. The responses are utilized to provide an indication of the capacitive load across the line which, in turn, may be translated to produce parameters indicative of, for example, line length, type of termination and possible line faults.
However, this prior art system possesses the same shortcomings and limitations summarized above with respect to the ARSB. Moreover, since the system is not comprised of a data base for storing information about line composition, adaptive testing and interpretation of results in view of line configuration information is precluded. In addition, although many users may have access to the system, each testing operation is basically sequential and there is no suggestion that access and testing operations on many different lines within one exchange may be occurring concurrently.
It is clear from a perusal of the prior art portion of the ARSB set forth in FIG. 1 that each grouping of test trunks is served by only one FE computer. In the event of a FE computer outage, the Local Test Desk could, temporarily, satisfy user test requests. However, such reliance reduces system throughput and is inappropriate in a fully automated testing environment. Such a shortcoming is obviated in an architecture that allows a plurality of FE computers to access any particular trunk.
To mitigate these and similar shortcomings, some distributed computer systems require that a cluster of controlling minicomputers communicate with remote entities that typically include microprocessor-based subsystems. However, when the number of such remote entities become large, a significant amount of minicomputer processing time must be devoted to these communication needs, and throughput is again reduced.
Also, packet switching networks may be used advantageously in some situations, but delay times through such networks and the cost of additional remote circuitry can render these solutions unattractive.
With the development of microprocessors, which function autonomously, it becomes feasible to decentralize switching functions and thereby offload many controller computer communication activities to the actual point of switching. Such an arrangement is discussed in U.S. Pat. No. 4,285,037 issued to H. Von Stetten on Aug. 18, 1981; this disclosure is selected as representative of numerous distributed processor switching networks configured for intercomputer communication. In these networks, all distributed processors, generally microprocessors, are connected to one another via a common bus. Communication of messages in the transmitting and receiving directions occurs between the processors in the form of information blocks having address information. A central clock is provided under whose control respective processors are cyclically connected for the emission of an information block and all other processors are connected to the common data bus in the receiving mode. Only the receiver having the specified address then receives the desired message. The processors comprising the system receive information from and transmit messages to associated peripheral or interface devices. For instance, some processors may be coupled to terminals or memories, whereas other processors may connect to communication lines having different baud rate capabilities.
The major shotcomings of such an arrangement include the utilization of a common bus which precludes alternative routing in case of a bus failure and the sequencing procedure allowing only one bus talker at a time. During peak message transfer periods, such an allocation procedure could lead to blocking situations with concomitant throughput delays.
Also, the standard communication sequences between a sender and receiver over a bus are usually replete with segments of no activity on the bus. For instance, after the sender transmits a message, the receiver computes a check word while the sender remains idle. The receiver then returns an acknowledge/negative acknowledge status message. Once the transmitted message is accepted, the sender then determines the next activity while the receiver is now inactive. Techniques have been devised to improve the efficiency of transmission in this simple sender-receiver situation. One such improvement utilizes the time the sender is idle (during check word computation) to effect a determination of the next activity.
The inefficiency is magnified in the situation of a talker communicating with many listeners. During the period in which one receiver is computing a check word, the remaining receivers are idle. If a retransmission is necessary, the inefficiency is compounded. Part of the difficulty occurs because the accept-reject status of a total message is also formulated as a message and returned over the data leads of the bus. Moreover, in situations exemplified by the MLT system, wherein the message propagate time is of the same order as a check word computation, the sender is idle for a significant portion of each transmission activity. This is in contrast to the situation wherein the messages are considerably longer than the check word computations, so the percentage of time the sender is idle is small.
As alluded to above in the summary of ARSB deficiencies, automated testing of coin telephone systems has, in the past, presented severe implementation problems because of the special nature of the coin equipment. Other special loop situations, such as analysis of dial pulses or measurement of nonlinear devices like thermistors, have also presented basically insurmountable implementation difficulties with conventional automated test procedures and equipment. Fortuitously, however, technological advances recently occurred which now make it possible to solve these problems and difficulties. Advances in microcomputer, digital signal processing and measurement technology have provided the motivation for the development of versatile digital signal generators and digital analysis techniques, including digital filtering, which produce rapid and accurate measurements. Unfortunately, however, the majority of subscriber lines to be tested are still analog in nature and parameters of interest relate to the frequency-dependent characteristics of the lines. Therefore, a suitable transponder for interfacing the analog lines to the sophisticated digital processing techniques is still a fundamental necessity.
One such transponder arrangement developed for sensitive line current measurements is disclosed in U.S. Pat. No. 4,274,051 issued to J. Condon on June 16, 1981. The invention set forth in this reference utilizes a pair of magnetic structures to produce an output signal when the current on the loop is other than zero. However, because the line currents undergoing measurement with this arrangement were large in magnitude, the errors caused by differences in the hysteresis characteristics between structures were negligible and could be ignored. Such errors, particularly when measuring differential currents, prove to be critical and require compensation to insure accuracy and resolution over the broad operating range expected of the transponder in the digital processing environment.